1. Field of the Invention
The present invention relates to a microlens for an organic EL element, an organic EL element using the same, and manufacturing methods thereof.
2. Related Background Art
An organic electroluminescence element (organic EL element) has been used as a self-luminous element for an image display device, such as a display, and for a surface light source. Such an organic EL element is generally prepared by sequentially stacking a transparent electrode serving as an anode, an organic layer, and a metal electrode serving as a cathode on a transparent supporting substrate, such as a glass and a transparent plastic film. In such an organic EL element, by a voltage applied between the transparent electrode and the metal electrode, electrons supplied from the cathode and holes supplied from the anode are recombined at the organic layer. Then, when an exciton thus generated transitions from an excited state to a ground state, EL emission occurs. Light of the EL emission goes through the transparent electrode, and then is extracted to the outside from the transparent supporting substrate side. Here, in order to enhance the efficiency of such light-extraction, the provision of the so-called microlens on a light-extraction surface has conventionally been studied.
For example, Japanese Unexamined Patent Application Publication No. 2004-241130 (Document 1) discloses an organic EL element in which a transparent electrode, an emitting layer formed of an organic electroluminescence material, and a metal electrode are sequentially stacked on one surface of a transparent substrate, and a microlens is provided to the other surface of the transparent substrate. Moreover, Document 1 discloses a method for forming a microlens on the transparent substrate. The method comprises the steps of: fabricating a mold of the microlens by applying polysilicon onto an outer surface of a silica glass plate that is to be the mold, and performing wet-etching on one surface (lower surface) of the silica glass plate for providing holes at positions to be portions in which convex shapes of the lens are formed; molding a resin into a shape of the mold by applying a thermosetting resin with a predetermined thickness onto one surface (upper surface) of a transparent substrate (glass plate), pasting the mold to the resin in a vacuum in such a way that no bubbles are trapped between the mold and the resin, and then curing the resin by irradiation of light such as UV from the other surface (lower surface) side of the transparent substrate; and forming the microlens on the transparent substrate by detaching the mold from the cured resin. Meanwhile, another method for manufacturing a microlens is disclosed by a paper authored by S. Moller et al., “Improved light out-coupling in organic light emitting diodes employing ordered microlens arrays (Document 2),” described on pages 3324 to 3327 of “JOURNAL OF APPLIED PHYSICS, Vol. 91, No. 5” issued in 2002. This method comprises the steps of: forming a mold by performing etching on a layer formed of SiNx on a silicon wafer using photolithography to form a hole pattern, and by then performing wet-etching on the silicon wafer using the layer formed of SiNx as a mask, and then removing the layer formed of SiNx; and forming by use of the mold a microlens formed of polydimethylsiloxane on a glass substrate by filling a precursor of polydimethylsiloxane between the mold and the glass substrate, then curing the precursor, and then detaching the mold. However, organic EL elements comprising the conventional microlens as described in Documents 1 and 2 are not necessarily satisfactory because of a high angle-dependence of an intensity of an emission spectrum, and a great change in chromaticity.